The combustion temperatures and pressures of automobile engines have been becoming increasingly higher to achieve higher power and lower fuel consumption. Accordingly, demand is increasingly mounting particularly in diesel engine pistons for higher heat resistance such as high-temperature yield strength, high-temperature rigidity, thermal cracking resistance, etc., and for lighter weight to achieve higher power and lower fuel consumption. Also, in order that there are no abnormal wear, scuffing, seizure, etc. in sliding portions, for instance, between a skirt portion of a piston and a cylinder liner, between a pin boss portion of a piston and a piston pin, between piston ring grooves and piston rings, etc., improvement is required on such properties as wear resistance, seizure resistance, low thermal expansion, etc. Particularly when the seizure resistance, which may be called “scuffing resistance” or “scoring resistance,” is low, a piston and/or a mating member are scratched, resulting in accelerated wear, and sometimes scuffing or seizure. Therefore, the seizure resistance is an extremely important property for the piston.
Aluminum alloys such as JIS AC8A, etc. have conventionally been used for diesel engine pistons for weight reduction. However, because aluminum alloy pistons have as low thermal and mechanical durability temperatures as about 350° C., and large thermal expansion, they suffer a problem that seizure and scuffing are likely to occur. Accordingly, spheroidal graphite cast iron having relatively high durability up to about 400° C., and good seizure resistance due to the self-lubrication of graphite in the microstructure has recently become adopted in place of aluminum alloys (see, for instance, JP 10-85924 A).
Despite sufficient ductility, however, pistons made of spheroidal graphite cast iron have insufficient heat resistance when their temperatures become 450° C. or higher, causing the problem that thermal cracking occurs in lips, etc. by repeated application of thermal and mechanical loads. Though the spheroidal graphite cast iron exhibits relatively good seizure resistance due to the self-lubrication of graphite up to a combustion pressure of about 15 MPa, its seizure resistance due to graphite lubrication becomes unsatisfactory, and its high-temperature yield strength and high-temperature rigidity decrease, when the combustion pressure is elevated to 20 MPa or more. As a result, a piston and a mating member (cylinder liner, etc.) are worn by their strong contact, resulting in large blowby. In addition, troubles such as scuffing, seizure and breakage occur due to uneven contact, etc., making it likely that the engine performance is deteriorated.
When the piston of spheroidal graphite cast iron is made thinner for weight reduction, its high-temperature rigidity becomes too low, so that cracking is likely to occur in a pin boss portion, a skirt portion, etc. in addition to the lip. Thus, the piston of spheroidal graphite cast iron cannot be made drastically light in weight.
To cope with the increased combustion temperature and pressure, U.S. Pat. No. 5,136,992 proposes a piston comprising a head portion having a pin boss portion, and a skirt portion, which are produced separately and integrally assembled. FIG. 9 is a cross-sectional view showing an example of such piston 100. The piston 100 comprises a head portion 101 comprising a combustion chamber 105, a top surface 106 and an opening fringe (lip) 107 of the combustion chamber 105, a skirt portion 102, a top land 108, ring grooves 109 for receiving piston rings, a pin boss portion 104, and a cooling hollow portion 103 called cooling channel or gallery, in which oil is circulated to cool the combustion chamber 105. 100hdenotes a distance (compression height) from a center of a hole for receiving the pin to the top surface 106.
The head portion 101 and the pin boss portion 104, which need high heat resistance, are formed by a precipitation-hardened, forged ferrite-pearlite steel comprising, by weight, 0.32-0.45% of C, 0.4-0.9% of Si, 1.0-1.8% of Mn, 0.035% or less of P, 0.065% or less of S, and 0.06-0.15% of V, the balance being Fe, and the skirt portion 102 is formed by a light alloy such as aluminum, etc. It is described that such microstructure makes it possible to produce a piston at a lower cost than a conventional FebalCr42Mo4 alloy (corresponding to JIS SCM440).
Though the forged steel piston 100 has excellent high-temperature rigidity, it is likely to have insufficient seizure resistance and wear resistance at a combustion pressure elevated to 20-25 MPa, because of no self-lubricating graphite in the microstructure. Also, because it is produced by a forging method, sulfides and non-metallic inclusions are extended thin and long in a main deformation direction (along a metal flow line) during forging, so that thermal cracking is likely to occur in the lip 107 of the combustion chamber 105, etc. under a high thermal and mechanical load.
In addition, it needs the step of assembling the head portion 101 and the skirt portion 102, resulting in a high production cost. Further, because it needs a space, into which a bite for machining the cooling hollow portion 103 is introduced, it inevitably has a large compression height 100h, resulting in difficulty in size reduction. Further, because the piston comprising the cooling hollow portion 103 cannot be produced integrally by one step in the forging method, the step of machining the cooling hollow portion 103, the step of fixing a cover f to the hollow portion 103, etc. are necessary, resulting in increased production cost.
Japanese Patent 2,981,899 proposes a piston ring subjected to nitriding to improve wear resistance and seizure resistance, which comprises, by weight, 0.6-1.1% of C, 2.0% or less of Si, 2% or less of Mn, 10.0-18.0% of Cr, 0.5-4.0% of Mo and/or W (Mo+½ W), 0.05-2.0% of V and/or Nb (V+½ Nb), 2.5% or less of Ni, 12% or less of Co, 0.5% or more of Ni+Co, 0.015% or less of P, 0.005% or less of S, and 30 ppm or less of 0, the balance being Fe and inevitable impurities, thereby having excellent corrosion wear properties and fatigue properties in a sulfuric acid atmosphere. It is described that V and Nb make crystal grains finer to improve toughness, form carbides to improve wear resistance and seizure resistance, and improve temper-softening resistance.
However, the piston ring, which is constituted by a narrow plate worked to a ring shape, can be easily produced even from steel containing a large amount of Cr carbides. However, when integral castings having complicated shapes and thus needing many machining steps, such as a piston, are produced, their production yield is low because of difficult casting and machining, needing a high cost, many steps, etc. It is thus extremely difficult to integrally cast a piston from the above piston ring material. Further, because the above piston ring material does not have heat resistance such as high-temperature yield strength, high-temperature rigidity and thermal cracking resistance, seizure resistance, etc. to levels needed for the piston, it cannot be used for an integrally cast piston.
It is expected that the piston temperature is elevated to about 450-500C, and the combustion pressure increases to about 20-25 MPa particularly in a diesel engine piston, as the combustion temperature is elevated. Accordingly, the piston should be resistant to such high temperature and pressure. In addition, the piston should have high seizure resistance, in order that scuffing, seizure, etc. do not occur by contact with a mating member such as a cylinder liner, a piston pin, piston rings, etc. during sliding under high thermal and mechanical load conditions. For higher power and lower fuel consumption of an engine, there are demands to reduce inertia during the reciprocal movement of a piston, to reduce the weight of a piston, to reduce the friction of a piston, to reduce engine noise, to downsize an engine, etc. It is thus desired to make the piston thinner, reduce the compression height, etc.
Integral casting to a near-net shape makes the assembling and connection of components unnecessary, and reduces the working cost. Accordingly, the integrally cast piston advantageously enjoys an extremely lower production cost than the assembled, forged piston of U.S. Pat. No. 5,136,992, which requires the machining of a cooling hollow portion, the fixing of a cover to the hollow portion, and the assembling of a head portion to a skirt portion. In addition, because the integrally cast piston does not need a space for machining the cooling hollow portion, it can have a low compression height, so that it can be made lighter and smaller. The integral casting of a head portion with a pin boss portion and a skirt portion can produce a gasoline engine piston needing no cooling hollow portion. Further, integral casting including a cooling hollow portion can produce a piston suitable for diesel engines. It is particularly suitable for a direct injection diesel engine piston comprising a combustion chamber in a head portion, and a cooling hollow portion near the combustion chamber.
In addition, the piston is required to have high strength and ductility, in order that no cracking and breakage occur by vibration and impact even when used under a high thermal and mechanical load. Particularly to avoid the generation of cracking and breakage, ductility is required not only in use in an engine, but also in a production step, an assembling step, etc. A low-temperature ductility generally at room temperature or lower is represented by a room-temperature elongation.